Snap-acting thermo-responsive member that minimizes creep action



March 1965 H. F. MALONE ETAL 3, ,925

SNAP-ACTING THERMO-RESPONSIVE MEMBER THAT MINIMIZES CREE? ACTION Filed May 1, 1961 2 Sheets-Sheet l INVENTORS.

WEE/5 ic-M,

United States Patent "ice This invention relates generally to snap-acting thermoresponsive members and more particularly to a snapact-ing therrno-responsive member and mounting therefor that minimizes pro-snap creep.

Snap-acting t-hermo-responsive members are often incorporated into electrical switches to effect interruption of an electric circuit due to an overload current therein or in response to a selected thermal condition. For example, such snap-acti11g thermo-responsive members are often incorporated into motor protectors to protect against a particular thermal condition resulting from excess current flow. Snap-action of the thermo-responsive member results in clean contact make and break, which insures long contact life and stable temperature control. Such thermo-responsive members must be capable of accurate and consistent calibration and preferably are relatively inexpensive and easy to fabricate.

The class of motor protectors to which the instant in vention has particular application may be termed proximi-ty, on the winding, in the Winding, or tubular motor protectors. These terms are descriptive of devices which are generally longer than they are wide and which have dimensional characteristics dictated by space limitations in certain types of motors. Such space limitations also require the thermo-responsive members to be rela tively long and narrow.

Previously, it has been impossible to obtain maximum Work out of such a long and narrow t-hermo-responsive member since it has been customary in the past to impress a spherical deformation in a relatively small part of the free length of the member. Necessarily, the plane diameter of the spherical deformation could be no greater than the width of the member.

The present invention is directed .to a concept of obtaining work from the entire length of the thermo-responsive members employed in protectors of the classes to which the previously used generic terms apply. The invention contemplates a new configuration for the stressed zone of the bimetal.

This internally stressed section is greater in length than in width and can be described as that of a frustrum of a prolate spheroid. In physical appearance it is similar to a section along the sideof a football parallel to its major axis. Such a section can be mathematically and practically designed to include substantially all of the deformable area bet-ween the contact and the anchored ends of the thermo-responsive member.

Characteristically, snap-acting thermo-responsive members heretofore known and used have been subject to initial creep of the free or unsupported portion thereof prior to snapping to a desired configuration. Such creep is generally undesirable, as in the aforementioned applications where a movable contact is secured to the free end of the thermo-responsive member, in that creep re duces contact pressure between the movable and fixed contacts. A reduction in contact pressure, with its in- I herent heating of the contacts, becomes critical when creepage is sufiici-ent to effect separation or incipient opening and closing of the contacts.

It has been proposed, in snap-acting t-hermo-responsive members heretofore known and used, to solve the aforementioned problem by employing a plura ity of ribs or cutouts in the therrno-responsive member thereby to stiff- 3,171,925 Patented Mar. 2, 1965 en the member in selected areas to control the flexure thereof upon heating. However, the provision of such ribs or cutouts is generally unsatisfactory since the response characteristic of the thermo-responsive member is often rendered inconsistent due to the tolerance problems inherent in an embossing or cutting operation. It has also been proposed to \apply pressure directly to the top of the impression in the thermo-responsive member on the convex side thereof to aid in the reversal of the internal stresses that eifect snap action. However, this expedient is not completely satisfactory as it reduces the calibration range of the thermo-responsive member.

The present invention provides a novel solution to the problem of creep in a snap-acting thermo-responsive member by substantially entirely internally pre-stressing the member between the supported and free ends thereof whereby minute inputs of energy supplied by initial 1 R losses or gains or by a temperature increase or decrease, are substantially entirely used in overcoming the internal pro-stress in the member, thereby minimizing pre-sn'ap creep. The thermo-responsive member is supported in a novel manner to minimize the unsupported area thereof and to maximize contact pressure without deleteriously affecting snap action.

Therefore, the broad object of the present invention is an improved snap-acting thermo-responsive member.

Another object of the present invention is an improved snap-acting thermo-responsive member wherein substantially all of the energy input is absorbed in effecting a snap action.

Another object is a thermo-responsive member having a mini-mum creep upon the occurrence of minute input of energy thereto.

Another object is an improved support for a dished thermo-responsive member.

Other objects and advantages of the present invention will be apparent in the following description, claims and drawings wherein:

FIGURE 1 is a sectional side elevational view of a snap-acting thermo-responsive member shown in operative association within a motor protector;

FIG. 2 is a top view of the back-up member and thermo-responsive member of FIGURE 1;

FIG. 3 is an exploded perspective view of the sandwiched components of FIGURE 1;

FIG. 4 is a top view of a modified motor protector with a portion of an exteriorly insulating sleeve removed therefrom to show a conductive inner shell;

FIG. 5 is a cross-sectional view taken substantially along the line 5-5 of FIG. 4;

FIG. 6 is a cross-sectional view taken substantially along the line 66 of FIG. 5;

FIG. 7 is a fragmentary view of the stressed portion of a thermo responsive member in accordance with the present invention;

FIG. 8 is a cross-sectional view taken substantially along the line 88 of FIG. 7; and

FIG. 9 is a crossasectional view taken substantially along the line 99 of FIG. 7.

A snap-acting thermo-responsive member or bimetal 10, in accordance with an exemplary embodiment of the present invention, is shown operatively associated with a motor protector 12;. The motor protector 12 comprises an elongated conductive outer shell 14 with a fixed contact 16 disposed interiorly thereof that is electrically connected to the case 14. An insulating end plug 18 of, for example, epoxy resin, sealably closes an open end 19 of the case 14. An outer end portion 20 of a terminal 22 is supported by the plug 18, the terminal 22 being electrically connected to a lead-in wire 24 adapted to be connected to one side of a source of electrical energy.

The terminal member 22 extends inwardly of the case 14, an inner end portion 28 thereof defining a back-up member that is held in pressure contact with one end 30 of the thermo-responsive member 10. The bimetal 10 and the inner end 28 of the terminal 22 are stacked between a pair of insulating members 32 and 33, a center portion 34 of the insulating member 32 being pushed through apertures 35, 36 and 37 in the bimetal 10, terminal member 22 and insulating member 33 to hold the assembly together in a sandwich construction.

A lead-in wire 40 having a terminal 42 thereon, is electrically connected to the case 14 of the switch 12 for connection thereof to the other side of a source of the electrical energy (not shown).

A movable contact 5b is secured to a free end portion 52 of the bimetal 11), as by welding, in operative alignment with the fixed contact 16 so as to be engag'eable therewith to complete an electrical circuit through the protector 12.

In accordance with one feature of the present invention, the inner end portion 28 of the terminal 22 is of forked construction and functions as a back-up or pressure plate having outer edge portions 54 and 56 extending from between the stacked insulating members 32 and 33 along and to a point substantially intransverse alignment with the minor axis of an embossment or nondevelopable surface 61), which can be described as a frustrum of a prolatespheroid, on the bimetal it Therefore, substantially the entire unembossed portion of the bimetallic member 11% between the outer end portion 30 and contact thereon is rigidly supported. The major axis of the embossment '60 extends longitudinally substantially the full unsupported length of the bimetal 10 between the apex 62 of the forked end portion 28 on the terminal 22 and the contact St} on the bimetal 10. Further, the minor axis of the embossment 6!) extends laterally substantially the full width of the bimetal 1d between the edge portions 54 and 56 on the inner end portion 28 of the terminal 22. Therefore, because substantially all of the unsupported area of the bimetal 10 between the fixed end portion 3% and the movable contact 5% is pro-stressed, which pre-stress must be overcome byinitial variations in temperature of the bimetal prior to snap-action thereof,

relatively small temperature changes effected by small 1 R gains or losses or thermal conditions operate only to change the pre-stressed condition of the bimetal as opposed to effecting the creep thereof. In this manner,

deleterious bimetal creep is substantially eliminated.

Referring now to FIGS. 4 through 6, a modified motor protector 112 comprises an elongated conductive shell 114 that is encapsulated within a plastic sleeve 115, made from, for example, Mylar. A fixed contact 116 is disposed interiorly of the shell 114 and is electrically connected to a terminal strip 117. An end portion 118 of the sleeve sealably closes an open end 11? of the shell 114. An outer end portion 12tlof the terminal strip 11? is electrically connected to a lead-in Wire 124, which is adapted to be connected to one side of a source of electrical energy.

As best seen in FIG. 5, a thermo-responsive bimetal has an end-portion 126 stacked between an insulating member 128 and a conductive member 131). An insulating member 132 is pushed through apertures 133, 134, 135, 138, and 139 in the conductive member 1319, bimetal 125, insulating member 128, terminal strip 117 and a lower insulator 141 to hold the assembly together in a sandwich'construction.

A lead-in wire 141) is electrically connected to the shell 114 for'connection thereof to the other side of a source of the electrical energy '(not shown).

A movable contact 150 is secured to a free end portion '151 of the bimetal 125, as by welding, inoperative alignment with the fixed contact 116 so as to be engageable therewith to complete an electrical circuit through the motor protector 112 running from the conductor 124, to the terminal strip 117, fixed contact 116, movable contact 4 150, bimetal 125, conductive member 130, shell 114, to the conductor 140.

In accordance with one feature of the present invention, an inner end portion 152 of the conductive member is of forked construction and functions as a back-up or pressure plate having outer edge portions 154 and 156 extending along the bimetal-125 toward-s the minor axis of an embossment or non-developable surface 166. The embossment 1643 can be described as a frustrum of a prolate spheroid. Therefore, substantially the entire unembossed portion of the bimetallic member 125 between the end portion 126 and contact thereon is rigidly supported. The major axis of the embossrnent 150 extends longitudinally substantially the full unsupported length of the bimetal 125 between an apex 162 of the forked end portion 152 of the member 130 and the contact 150 on the bimetal 125. Further, the minor axis of the embossment 160 extends laterally substantially the full width of the bimetal 125 between the edge portions 154 and 156 on the inner end portion 152 of the conductive member 130. Therefore, because substantially all of the unsupported area of the bimetal 125 is prestressed, which pre-stress must be overcome by initial variations in temperature of the bimetal 125 prior to snapaction thereof, relatively small temperature changes effected by small 1 R gains or losses or thermal conditions operate only to change the pre-stressed condition of the bimetal 125 as opposed to effecting creep thereof. In this manner, deleterious bimetal creep is substantially eliminated.

It is to be noted that the forked end portion 152 is used as a means of assuring a known initial contact pressure and of defining the point about which the bimetal 125 hinges when a temperature is reached which causes the stressed area 160 to pass through its unstable zone from a convex to a concave shape.

Referring to FIGS. 7 through 9, the prolate hemispherical embossment 150 is embossed into the bimetal 125 as by a pair of dies 200 and 202 having convex and concave sections 21M and 206,.respectively. It is to be noted that, as seen in FIG. 8, sections of the bimetal 125 along the major and minor axes of the embossment 160 are arcuate and are developed by radii 2% and 21%, respectively. It has been found that the embossment 160, when developed in this manner, exhibits optimum snap-acting characteristics.

It is to be understood that the specific constructions of the improved motor protector and snap-acting thermoresponsive member herein disclosed and described are presented for the purpose of explanation and illustration and are not intended to indicate limits of the invention, the scope of which is defined by the following claims.

What is claimed is:

1. An elongated generally rectangular snap-acting thermo-responsive bimetallic member having a free end portion and a fixed end portion with a non-developable surface extending therebetween and substantially entirely thereacross, and a supporting'member spaced from the non-developable surface of said bimetallic member and overlaying substantially all of the area of the bimetal between said -non-developable surface and the fixed end thereof to minimize pro-snap creep of said thermo-responsive member.

2. An elongated generally rectangular snap-acting thermo-responsivemember having mounting means at one end and a contact at the other end with a generally elliptical non-developable surface extending substantially entirely across the width thereof, said non-developable surface being the sole means for effecting snap action of the member, and a supporting member having a forked configuration at one end-thereof extending in close proximate relation to one end of said thermo-responsive member defined by a minor axis of its generally elliptical configuration, said supporting member being spaced from the nondevelopable surface of said therrno-responsive member r' i V 5 '3 but engaging and directly supporting substantially all of spheroid with the major axis thereof extending generally the area thereof between the mounting means and said parallel to the lengthwise dimension of said member.

non-developable surface thereby to minimize pre-snap References Cited in the file of this patent creep of said thermo-responslve member.

3. A snap-acting thermo-metal member in accordance 5 UNITED STATES PATENTS with claim 1 wherein said member has a contact at said 2,317,831 Vaughan Apr. 27, 1943 free end having a greater length than Width between Said 2,820,870 Moksu I an. 21, 195 8 free end and said fixed end, said non-developahle surface 2,860,208 Epstein Nov. 11, 1958 having the general contour of a frustrum of a prolate 3,004,203 Epstein Oct. 10, 1961 

1. AN ELONGATED GENERALLY RECTANGULAR SNAP-ACTING THERMO-RESPONSIVE BIMETALLIC MEMBER HAVING A FREE END PORTION AND A FIXED END PORTION WITH A NON-DEVELOPABLE SURFACE EXTENDING THEREBETWEEN AND SUBSTANTIALLY ENTIRELY THEREACROSS, AND A SUPPORTING MEMBER SPACED FROM THE NON-DEVELOPABLE SURFACE OF SAID BIMETALLIC MEMBER AND OVERLAYING SUBSTANTIALLY ALL OF THE AREA OF THE BIMETAL BETWEEN SAID NON-DEVELOPABLE SURFACE AND THE FIXED END THEREOF TO MINIMIZE PRE-SNAP CREEP OF SAID THERMO-RESPONSIVE MEMBER. 